The present invention relates to a friction reduction technology for machine parts which aims at reducing friction by forming microscopic recesses on a sliding contact surface of the machine parts. Specifically, this invention relates to a low friction element capable of reducing friction loss in engine parts for automobiles.
There has been proposed a surface microstructure in which microscopic recesses or grooves are formed on a sliding contact surface of a sliding element slidable relative to a counterpart via lubricating oil, in order to reduce friction caused therebetween.
In the surface microstructure of the sliding contact surface of the sliding element in an earlier technology, microscopic recesses or grooves have a uniform depth. In this case, when a great change in condition of friction between the sliding contact surface of the sliding element and the corresponding sliding contact surface of the counterpart, for instance, sliding velocity of the sliding element or shear rate of the oil film formed between the sliding contact surfaces, occurs at the sliding contact surface that comes into sliding contact with the corresponding surface of the counterpart upon the sliding motion of the sliding element relative to the counterpart, oil retention capacity and oil film thickness on the sliding contact surface cannot be optimally controlled. Specifically, in the case of a reciprocating sliding element, at a mid-point of the sliding stroke in which the sliding velocity becomes high, a lubricating oil film formed on the sliding contact surface is subjected to shear strain at high speed at microscopic projections formed between the recesses. This causes shear loss in the oil film to thereby increase friction loss. Further, the sliding velocity becomes zero at both ends of the sliding stroke, namely, a turning point of the reciprocating motion of the sliding element. Therefore, at the both ends of the sliding stroke, the lubricating oil is prevented from being brought onto the sliding contact surface and allowed to run away through the recesses. This causes lack of the oil on the sliding contact surface, so that the effect of friction reduction is restricted. There is a demand to eliminate the above-described problems in the earlier technology. Meanwhile, it has been considered that in a case where a piston has a smooth sliding contact surface and a cylinder wall has a smooth sliding contact surface defining a cylinder bore, friction caused therebetween will be restricted to the minimum, while the oil retention property of the smooth sliding contact surfaces will be deteriorated to thereby cause seizure of the smooth sliding contact surfaces.
An object of the present invention is to provide a low friction sliding element capable of reducing friction loss at a mid-point of the sliding stroke and preventing lack of lubricating oil at both ends of the sliding stroke, which can be utilized for reciprocating parts of engines, for instance, a piston, a cylinder wall and a valve lifter.
According to one aspect of the present invention, there is provided a low friction sliding element for cooperating with a counterpart to make a relative sliding motion, the sliding element comprising:
a sliding contact surface having microscopic recesses and plateaus interrupted by the recesses, the sliding contact surface being adapted to be in sliding contact with the counterpart during the relative sliding motion,
the recesses having depths regularly varying in a predetermined direction.
Preferably, each of the plateaus of the sliding contact surface of the sliding element of the present invention has a surface having an arithmetical mean roughness Ra of not more than 0.3 xcexcm. The arithmetical mean roughness Ra may be not less than 0.01 xcexcm. The arithmetical mean roughness Ra is prescribed in JIS B 0601-1994. The sliding contact surface of the sliding element is adapted to be lubricated with a lubricating oil. Further, a ratio of a maximum depth of the recesses of the sliding contact surface of the sliding element to a minimum depth of the recesses thereof may be not less than two.
The predetermined direction in which the depths of the recesses regularly vary may be a sliding direction of the sliding element. The recesses of the sliding contact surface of the sliding element of the present invention may be in the form of grooves extending perpendicular to the sliding direction of the sliding element. The recesses and the plateaus have widths extending on a center line of a roughness curve of the sliding contact surface, respectively. A ratio of the width of each recess to the width of each plateau may be not less than {fraction (1/10)}. The ratio of the width of the recess to the width of the plateau may be within a range of ⅕ to 5 at a predetermined portion of the sliding contact surface in which maximum sliding friction is caused. The predetermined portion of the sliding contact surface corresponds to a mid-point of a stroke of the sliding element. Further, a ratio of a maximum depth of the recesses to a thickness of the lubricating oil film formed on the plateaus is preferably within a range of 2 to 10 at the predetermined portion of the sliding contact surface. The maximum depth of the recesses may be within a range of 5 to 30 xcexcm. The depths of the recesses may be maximum at the predetermined portion of the sliding contact surface, and be gradually decreased as the recesses are further apart from the predetermined portion of the sliding contact surface. Further, the recesses may be formed between opposed end portions of the sliding contact surface which are located in the predetermined direction, and the depths of the recesses may be gradually increased from near the opposed ends of the sliding contact surface toward the predetermined portion of the sliding contact surface. The opposed end portions of the sliding contact surface may correspond to both ends of the stroke of the sliding element.
Further, a sliding contact surface of the counterpart may have an arithmetical mean roughness Ra of not more than 0.3 xcexcm. The arithmetical mean roughness Ra of the sliding contact surface of the counterpart is not less than 0.1 xcexcm. Furthermore, the sliding element of the present invention may be applied to engine parts including a piston skirt for an engine, an engine cylinder wall defining a cylinder bore, a cylinder liner for an engine cylinder, and a valve lifter for an engine valve.